In recent years, attention has been paid to a self light emitting display using a light emitting device as a next generation display. Among other things, there has been known a display using an organic EL device, i.e., an organic EL display, being a current-control light emitting device whose light emitting brightness is controlled by current. As the organic EL display, there is an active-matrix organic EL display using a thin film transistor (TFT) in its display area and peripheral circuit. As one of its driving systems, there has been used a current programming method in which the magnitude of current corresponding to image data is set to a pixel circuit formed in a pixel to emit the organic EL device.
FIG. 9 is one example of the configuration of a pixel circuit in a conventional current programming method including an organic EL device. In FIG. 9, scanning signals P1 and P2 and current data Idata as a data signal are input to the pixel circuit. The anode of the organic EL device is connected to the drain terminal of a TFT (M4) and the cathode of the organic EL device is connected to a ground potential CGND. P-type TFTs M1, M2 and M4 and N-type TFT M3 are included in the pixel circuit.
FIG. 10 is a timing chart illustrating the drive operation of a pixel circuit 2. Idata denotes a current data. That is to say, I(i−1), I(i) and I(i+1) illustrate current data Idata input to the pixel circuits 2 in the subject column in an (i−1) row (or, a row preceding by one row with respect to a subject row), “i” row (or, a subject row) and (i+1) row (or, a row following the subject row). P1 and P2 are scanning signals.
Before a time t0, a signal with a low level is input as the scanning signal P1 to the pixel circuits 2 in a subject row and a signal with a high level is input as the scanning signal P2 thereto. The transistor M2 is turned off, the transistor M3 is turned off and the transistor M4 is turned on. In this state, the current data I (m−1) corresponding to the current data Idata in the row preceding by one row is not input to the pixel circuit 2 in the “m” row being the subject row.
At the time t0, a signal with a high level is input as the scanning signal P1 and a signal with a low level is input as the scanning signal P2. The transistors M2 and M3 are turned on and the transistor M4 is turned off. In this state, the current data I (m) corresponding to the current data Idata in the corresponding row is input to the pixel circuits 2 in the “m” row. At this point, the M4 is not in the conductive state, so that current does not flow into the organic EL device. The input Idata produces a voltage according to the current drive capacity of the transistor M1 across a capacitor C1 arranged between the gate terminal of the M1 and a power source potential VCC.
At a time t1, a signal with a high level is input as the scanning signal P2. The transistor M2 is turned off. At a time t2, a signal with a low level is input as the scanning signal P1. The transistor M3 is turned off. The transistor M4 is turned on. In this state, the transistor M4 is in the conductive state, so that the voltage produced across the capacitor C1 supplies the organic EL device with current according to the current drive capacity of the transistor M1. This causes the organic EL device to emit light having a brightness according to a supplied current.
However, a current flowing into the organic EL device in one pixel is minute and, in particular, the current data Idata for causing the device to emit light having a low brightness is extremely small. For this reason, it takes a very long time to charge a data line at the time of programming a desired current, so that one scanning period (or, a period for which the scanning signal P2 is in a low level from the time t0 to the time t1) is too short to charge the data line. Then, there has been known a duty driving in which a comparatively large current is set to a pixel circuit to control a light emitting period, thereby controlling a brightness, however, the flicker is produced unless the pixel circuit is driven at a high frequency to some extent.
For this reason, U.S. Patent Application Publication No. 2005/0007319 proposes a display apparatus in which a light emitting period is controlled by the duty driving while a display is being performed by the interlace method in which one frame is formed of two fields (or, odd and even fields).
FIG. 11 is a timing chart describing a method of driving the display apparatus disclosed in the above document. In FIG. 11, one frame (“1 frame” in the figure) is formed of an odd field (or, “ODD field” in the figure) and an even field (or, “EVEN field” in the figure). A row number of the display apparatus is represented by 1 to m.
In FIG. 11, X(1) to X(m) represent scanning signals corresponding to each row and select rows when they are in a high level state to perform a current programming. Z(1) to Z(m) denote lighting signals corresponding to each row. A pixel emits light when the signal is in a low level state, but it does not emit when the signal is in a high level state. In the odd field, only odd rows are selected to perform the current programming. In the even field, only even rows are selected to perform the current programming.
Thus, the control lines corresponding to odd lines and even lines are separated from each other and driven and the organic EL devices are duty-driven, thereby making the light emitting and the non-light emitting periods between adjacent lines different to remove the flicker.
For example, if one field is displayed at 60 Hz using the driving method of the above document, one frame is displayed at 30 Hz. That is to say, a driving frequency is 30 Hz at which light emitting and non-light emitting is repeated on a certain line, this frequency is not enough to prevent the flicker from being produced. As a result, an image quality is degraded.